Effect of Te and S Donor Levels on the Properties ofGaAs1−xPxnear the Direct-Indirect Transition

The effect of donor impurity levels associated with higher-lying conduction-band minima on the direct-indirect transition in heavily doped $\mathrm{Ga}{\mathrm{As}}_{1-x}{\mathrm{P}}_x$ has been studied. Hall-coefficient measurements from 55 to 400°K and resistivity measurements under hydrostatic pressure between 0 and 7 kbar at 300, 195, and 77°K have been made throughout the alloy composition range. The behavior of Te-doped crystals can best be explained by an impurity level ∼0.03 eV below and associated with the [100] minima. The pressure coefficient of this level with respect to the [000] minimum is about 10.5×${10}^{-3\mathrm{}}$ eV/kbar. Two impurity levels are found in S-doped crystals. The deeper of these two levels is ∼0.04 eV beneath the [000] minimum for crystal composition $x=0.30\mathrm{}$; the depth increases to ∼0.21 eV at $x=0.45\mathrm{}$. For further increase in $x$, this level becomes shallower. A persistent photoconductive effect is exhibited by this level at $T\le 100°$K. This behavior has been studied throughout the alloy composition range by means of photo-Hall measurements. The pressure coefficient of the deeper S level with respect to the [000] minimum is (10.8±0.3)×${10}^{-3\mathrm{}}$ eV/kbar. From this and from the effect of crystal composition on the dependence of resistivity on pressure, it is concluded that the deeper S level is associated with the [100] conduction-band minima. At 77°K a second S level is observed which at $x=0.30\mathrm{}$ is degenerate with the [000] minimum and has an estimated depth of 0.06 eV beneath the [100] minima. This level has a pressure coefficient of (10.0±0.4)×${10}^{-3\mathrm{}}$ eV/kbar with respect to the [000] minimum, and is observed at 77°K because of the inability of the deeper S level to accept additional electrons when $T\le 100°$K. The mobility ratio of the two bands, $\frac{\mu \mathrm{}\left(000\right)\mathrm{}}{\mu \mathrm{}\left(100\right)\mathrm{}}$, is 15-30 at 300 and 195°K but increases to 60-100 at 77°K. The phosphorus concentration $x$ at which the [000] and [100] minima are equal in energy is $x=0.43\mathrm{}\pm 0.01$ at 77°K and 0.45±0.01 at 300°K, implying that the separation between the minima increases with temperature at the rate of ∼6×${10}^{-5\mathrm{}}$ eV/°K.